Characterization of the Interfacial Toughness in a Novel "GaN-on-Diamond" Material for High-Power RF Devices

GaN thin film integrated to polycrystalline diamond substrates is a novel microwave transistor material with significantly improved heat dissipation capability. Due to the thermal and mechanical properties mismatch between GaN and diamond, a natural concern arises in terms of its interfacial stability as currently there is no established method to evaluate the interfacial toughness in GaN-on-diamond material. Using three generations of "GaN-on-Diamond" materials with varying process parameters, a comprehensive study has been carried out to identify the most appropriate fracture mechanics-based methods for reliable evaluation of the interfacial toughness in this novel material system. Several techniques were assessed, and the results are cross-compared; these include an ex situ nanoindentation induced buckling method and two-step indentation approach together with several analytical models. Additionally, a microcantilever bending method was adopted to measure an upper bound for the interfacial fracture toughness. For the three generations of materials, the interfacial toughness, G(Ic), was determined to be 0.7, 0.9, and 0.6 J.m(-2), respectively. Postmortem analysis of the micro- and nanostructure of fractured interfaces indicated that the systems with better heat spreading capability displayed smoother fracture surfaces, i.e., were more brittle due to the lack of active toughening mechanisms. Potential modifications to the interface for improved mechanical stability were proposed based on the experimental results.